1. Field of the Invention
The present invention relates to an ink jet device used in, for example, an ink jet printer.
2. Description of the Related Art
FIG. 1 shows a conventional ink jet head 100 used in an ink jet printer to eject ink droplets. The ink jet head 100 includes a chamber block 103 and a piezoelectric element 122. The chamber block 103 is formed with a pressure chamber 116, a manifold 124, and an ejection nozzle 120. The pressure chamber 116 is filled with ink. The piezoelectric element 122 is fixed on the upper wall of the chamber block 103 and is connected to a drive circuit 110. To eject ink droplets 126 from the ejection nozzle 120, the drive circuit 110 applies a voltage pulse to the piezoelectric element 122 so that the piezoelectric element 122 deforms. The upper wall of the chamber block 103 deforms accordingly as indicated by dotted line in FIG. 1. When the upper wall of the chamber block 103 deforms into the pressure chamber 116 in this manner, the pressure in the pressure chamber 116 increases and pushes ink out from the pressure chamber 116 and the nozzle 120 in the form of ink droplets 126.
In the example of FIG. 1, a drive voltage pulse waveform including three drive pulses is applied to the piezoelectric element 122, so that three ink droplets 126 are ejected in succession. The ink droplets 126 are connected together as shown in FIG. 1 when first ejected from the nozzle 120, but separate during flight. The simplest drive voltage pulse waveform applied by the drive circuit 110 to the piezoelectric element 122 is a single pulse drive waveform configured from only a single drive pulse.
Once the applied voltage reaches a peak value for ejecting the ink droplets 120, it returns to a base voltage whereupon the piezoelectric element 122 and the upper wall of the chamber block 103 return to their initial shape. The pressure in the pressure chamber 116 drops so that ink ejection stops. At the same time, ink is drawn into the pressure chamber 116 from an ink tank (not shown) through the manifold 124.
FIG. 2 shows an ink ejection head 101 that includes a plurality of pressure chambers 116 and nozzles 120. Piezoelectric elements 122 are provided on confronting walls that form the pressure chambers 116. Although not shown in the drawings, a head transport mechanism is provided for transporting the ink ejection head 101 in consecutive rows in a main scan direction, which is perpendicular to the direction in which the pressure chambers 116 and the nozzles 120 are aligned. Also, a sheet transport mechanism is provided for transporting sheets in an auxiliary scan direction, which is parallel to the direction in which the pressure chambers 116 and the nozzles 120 are aligned.
To print images on a print sheet, the head transport mechanism transports the ink ejection head 101 in the main scan direction while the drive circuit 110 applies drive voltage pulses to selective sets of the piezoelectric elements 122 in consecutive ejection operations of the same row. At the end of the first row, the sheet transport mechanism transports the print sheet in the auxiliary scan direction by a distance equivalent to the length of the ink ejection head 101 while the head transport mechanism moves the ink ejection head 101 back to its initial position. Then, the transport mechanism again transports the ink ejection head 101 in the main scan direction to print another row of images.
The inventor observed that images printed by the ink jet device 101 lack uniformity of image density during the first few millimeters after the ink jet device 101 begins printing a new row of images in the main scan direction. The inventor investigated this problem and discovered that when a plurality of nozzles are fired at the same time soon after start of an image row, abnormal residual pressure fluctuation patterns develop between the manifold that supplies ink into the ink chamber and the ejection nozzle. Further, the inventor discovered that the type of residual pressure fluctuation pattern depends on the number of nozzles that are simultaneously fired. These residual pressure fluctuation patterns influence the volume of ink droplets ejected simultaneously. The resultant dot column in the printed row can appear darker or lighter than surrounding dot columns, depending on whether the volume of the droplets was greater than or less than normal volume. This variation in density of adjacent dot columns can appear as vertical stripes in the first few millimeters of the printed image.
It is an objective of the present invention to overcome the above-described problem and provide an ink jet device that prints with uniform image density throughout the entire print row.
In order to achieve the above-described objectives, an ink ejecting device according to the present invention includes a print head, a scanning mechanism, a drive circuit, and a controlled. The print head includes a plurality of pressure chambers, actuators, and nozzles in a one-to-one correspondence with each other. Each pressure chamber is in fluid communication with a corresponding nozzle and filled with ink. The actuators generate energy upon application of voltage to eject ink from the corresponding pressure chamber through the corresponding nozzle. The scanning mechanism moves the print head to scan in consecutive rows. The drive circuit applies voltage to the actuators.
The controller controls the drive circuit to apply voltage selectively to the actuators to drive the print head to perform consecutive ejection operations while the scanning mechanism scans the print head in each row. The controller generates drive waveforms that compensate for pressure fluctuations in the pressure chambers, for each of a predetermined number of ejection operations from the start of each row that the scanning mechanism scans the print head and applies them to the drive circuit. The controller generates the drive waveforms depending on which of the predetermined number of ejection operations is to be performed next in a present row.
According to another aspect of the present invention, an ink ejecting device includes the print head, the scanning mechanism, and the drive circuit of the first aspect, and further includes a waveform memory, a compensation feature memory, and a different controller. The waveform memory stores a waveform. The compensation feature memory stores features, such as voltage values, pulse widths, and the like, that are used to modify the waveform in the waveform memory in order to compensate for pressure fluctuations in the pressure chambers.
The controller of the second aspect controls the drive. Circuit to apply voltage selectively to the actuators to drive the print head to perform consecutive ejection operations while the scanning mechanism scans the print head in each row. The controller selects a feature from the compensation feature memory for each of a predetermined number of ejection operations from start of each row that the scanning mechanism scans the print head. The controller selects the features depending on which of the predetermined number of ejection operations is to be performed next in a present row, and then uses the selected feature to modify the waveform into a modified waveform. The controller then controls the drive circuit based on the modified waveform for the corresponding ejection operation.
A method according to the present invention involves storing a waveform in a waveform memory and a table in a compensation feature memory. The table lists one type of features such as voltage, pulse width, and the like, in correspondence with each of a predetermined number of consecutive ejection operations from start of a scan row. The waveform stored in the waveform memory is modified based on the feature listed in correspondence with an ink ejection operation to be performed next in a scan row. Then voltage is applied to actuators of a print head based on the modified waveform.